Mold clamping apparatuses of an injection molding machine are classified into toggle, direct, and crank types in relation to a drive force transmission mechanism, and are classified into hydraulic and servomotor-operated types in relation to a drive force generating source.
In general, a hydraulic injection molding machine uses an operating fluid which is liable to be affected by the atmospheric temperature so as to expand and contract in volume. For this reason, a stable clamping force and repeatability are difficult to obtain. Moreover, complicated maintenance is required, and it is difficult to obtain clean molding products. In this respect, an advantage of a servomotor-operated mold clamping apparatus, without the need of using pressurized fluid, has been recognized.
In a servomotor-operated direct mold clamping apparatus, a complicated control is required for adjustment of the mold-opening stroke. It is difficult to produce a large mold clamping force because of the difficulty in changing to any significant degree magnitude of the drive force during the time between when mold-opening or mold-closing is effected and when mold clamping is effected.
A servomotor-operated toggle mold clamping apparatus is capable of producing a large mold clamping force by increasing the motor output by means of a toggle mechanism. Significant labor is required, however, for a lockup operation, and hence a high-speed injection molding cycle cannot be achieved.
On the other hand, a crank mold clamping apparatus is advantageous in that it has a simplified structure and is capable of carrying out the injection molding cycle at a high speed. However, this type of apparatus is arranged so that a moving platen assumes the mold-closing position and the mold-opening position when a crank reaches its top dead center and bottom dead center, respectively, during one revolution of the crank, and hence the mold-opening stroke of the apparatus is fixedly determined in dependence on the crank length. Thus, it is poor in versatility. For instance, when a three plate type of mold, having a stripper plate for ejecting a mold product, disposed for movement between mold halves which are attached to the front and movable platens, respectively, is employed in an injection molding machine having a fixed mold-opening stroke, only a particular mold is usable for a particular injection molding machine because an individual mold has its proper mold-opening distance. Therefore, this type of mold lacks usefulness in practical use unless it is employed in a mold clamping apparatus which is variable in its mold-opening stroke. In this respect, conventionally, the crank mold clamping apparatus having the above-mentioned advantages has been hardly utilized.